Aromatic Permeation through Crystalline Molecular Sieve Membranes

Membranes. Christopher J. Gump, Vu A. Tuan, Richard D. Noble, and John L. Falconer*. University of Colorado, Department of Chemical Engineering, Bould...
0 downloads 0 Views 285KB Size
Download PDF
Ind. Eng. Chem. Res. 2001, 40, 565-577

565

Aromatic Permeation through Crystalline Molecular Sieve Membranes Christopher J. Gump, Vu A. Tuan, Richard D. Noble, and John L. Falconer* University of Colorado, Department of Chemical Engineering, Boulder, Colorado 80309-0424

The fluxes of aromatic molecules (p-xylene, o-xylene, and benzene) were measured as a function of temperature and feed partial pressure through several molecular sieve membranes (SAPO-5, SAPO-11, and mordenite) and three types of MFI membranes (silicalite-1, ZSM-5, and boronsubstituted ZSM-5). Single-file diffusion appeared to control transport through the SAPO and mordenite membranes. Hence, those membranes showed ideal selectivities greater than 1 for benzene over the xylene isomers but no separation selectivities for the mixtures. Surface diffusion and activated gaseous transport were the controlling mechanisms for the MFI membranes. The highest p-xylene/o-xylene selectivities were obtained for a boron-substituted ZSM-5 membrane. At feed partial pressures of 2.1 kPa and at a temperature of 425 K, the best selectivities were 130 (ideal) and 60 (separation). Zeolite pores preferentially permeated p-xylene and took as long as 8 h to reach steady state. Nonzeolite pores preferentially permeated o-xylene after much shorter breakthrough times. Higher pressures of p-xylene distorted the membrane framework, resulting in increased o-xylene permeation and reduced selectivity. After reaching steady state, the flux of p-xylene through zeolite pores was stable for at least 10 h. The flux of o-xylene through nonzeolite pores was similarly stable at 373 K but continuously decreased for at least 12 h at 405 K. Introduction Molecular sieves are a class of crystalline materials with highly structured pores in the nanometer range. Zeolites are molecular sieves that contain only silicon, aluminum, and oxygen in the crystalline lattice. These structures, which often contain acid sites and adsorb some molecules strongly, have found wide use in industry as catalysts1 and selective adsorbents for separations.2,3 Several molecular sieve structures have been synthesized into membranes. These membranes consist of a thin layer of intergrown crystals and can separate mixtures on the basis of preferential or competitive adsorption, diffusion, or molecular sieving. Their inorganic crystalline structure gives them mechanical strength and thermal and chemical stability. Therefore, they have the potential for separating mixtures of compounds that are otherwise difficult to separate, including organic isomers. Also, because the various molecular sieve structures have different pore sizes, the choice of structure can be tailored to the desired separation. For the separation of xylene isomers (kinetic diameters of 0.585 nm for p-xylene and 0.685 nm for m- and o-xylene), large-pore molecular sieve structures are of interest. Much of the existing experimental work on zeolite membranes was performed using MFI crystals, including ZSM-5 and its pure silica analogue, silicalite-1. The MFI zeolite pore structure (Figure 1a) consists of straight, circular pores (0.54 × 0.56 nm) interconnected with sinusoidal, elliptical pores (0.51 × 0.54 nm), as measured by XRD.4 The MFI membranes have been used to successfully separate mixtures of butane, hexane, or xylene isomers, as well as several alkane and * Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: (303) 492-8005. Fax: (303) 492-4341.

light gas mixtures.5-8 More recently, MFI membranes have been modified by the isomorphous substitution of other metal atoms besides Al into the framework. Tuan et al.9 synthesized borosilicate MFI membranes. The incorporation of boron into the framework alters the adsorption, hydrophilicity, and acid strength characteristics of the resulting molecular sieve crystals and decreases the unit cell size of the MFI structure.10 Mordenite (MOR) membranes have been synthesized successfully by the vapor-phase transport method of Matsukata et al.11 This synthesis technique involves coating the support with a mixture of amorphous silica and alumina and then heating the support under an atmosphere of organic template and water. The MOR pore structure (Figure 1b) consists of an interconnected two-dimensional network of elliptical pores. Eightmembered rings make up the 0.26 × 0.57-nm pores, and 12-membered rings make up the 0.65 × 0.70-nm pores. However, when used to separate mixtures of larger molecules, only the larger pore is likely to contribute to transport through the membrane. Pure-component permeation of several light gases through mordenite membranes implies that both gas-phase and surface diffusion are important.12 Silicoaluminophospates (SAPO) are another common class of crystalline molecular sieves. Several SAPO structures with a wide range of pore sizes and catalytic properties have been synthesized.10 Only a few of these structures have been grown into continuous films or membranes.13-17 SAPO-5, an AFI analogue (Figure 1c), consists of a one-dimensional 0.73-nm pore structure formed by a 12-membered ring.18,19 Tsai et al.17 synthesized SAPO-5 membranes on anodic alumina supports using a microwave hydrothermal synthesis technique. Although they reported no permeation results, the membranes showed a high degree of orientation as determined by XRD and SEM analysis. Also, they were

10.1021/ie000553i CCC: $20.00 © 2001 American Chemical Society Published on Web 12/21/2000

566

Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001

Figure 1. (a) Two-dimensional framework and pore structure for MFI zeolites. (b) One-dimensional framework for mordenite. (c) Onedimensional framework for AFI. (d) One-dimensional framework for AEL.

able to control the crystal size and shape of the membranes by varying the silicon content of the gel and the synthesis times. The SAPO-11 pore structure (Figure 1d) is also one-dimensional but slightly smaller than SAPO-5. It is composed of a 10-membered ring in a 0.39 × 0.63-nm oval. No reports of membranes synthesized using this molecular sieve were found in the literature. The medium-strength acidity of SAPO-11 makes it an effective isomerization catalyst, with less of the cracking reaction occurring.20 Because only a thin layer of intergrown molecular sieve crystals is required to form an effective membrane, the crystals are usually deposited on a mesoporous support, which provides mechanical strength. The type of support used affects the properties of the finished membrane. Kusakabe et al.21 studied membranes grown on R- and γ-alumina tubes of varying pore sizes. The various supports exhibited different permeation behavior, but no direct relationship between support and permeation could be identified. Coronas et al.22,23 found that the performance of membranes prepared on R- and γ-alumina tubes by hydrothermal synthesis depended on both the support and the synthesis procedure.

Flanders et al.8 found that, although the mixture selectivities were similar for alumina- and stainlesssteel-supported membranes, the ideal selectivities were an order of magnitude larger for the stainless-steelsupported membranes. However, no definitive correlations have been found concerning the effect of support type on zeolite membranes. Xylene isomers are challenging to separate because they have similar physical properties. High-purity pxylene is required for the production of synthetic fibers, polyester films, and resins. However, distillation of the xylene isomers is not economical because of their similar vapor pressures. Instead, separations are accomplished either by preferential adsorption or by fractional crystallization. Both techniques are batch processes and energy- and equipment-intensive. Therefore, a continuous separation process is desirable. Initial attempts at using pervaporation through membranes focused on polymers containing side groups that interacted strongly with one of the xylene isomers.24-28 However, the selectivities were too low (20

38

60

this work

pervaporation, 303 K MFI

pervaporation, 299 K pervaporation, 299 K vapor permeation, 380 K, 5 kPa feed pressure vapor permeation, 473 K, 0.3 kPa feed pressure vapor permeation, 373 K, 0.3 kPa feed pressure vapor permeation, 373 K, 0.05 kPa feed pressure vapor permeation, 425 K, 2.1 kPa feed pressure

recently, poly(vinyl alcohol) membranes containing cyclodextrin were studied by Miyata et al.30 When the permeants were fed to the membrane as vapors generated directly from the liquid (no helium diluent), a 10% p-xylene in o-xylene mixture was separated with a selectivity of about 4. Because polymer membranes have not proven successful at separating xylene isomers, the use of zeolite and other molecular sieve membranes has been explored. Table 1 summarizes some of the aromatic permeation work performed by other researchers using a variety of molecular sieve membranes. Nishiyama and co-workers12,31-33 studied the pervaporation of aromatics through ferrierite and mordenite membranes synthesized by the vapor-phase transport of Matsukata et al.11 They were able to separate benzene/p-xylene mixtures with selectivities of over 160 using a mordenite membrane and concluded that shape selectivity at the pore entrances of the mordenite structure controlled the separation. Using a ferrierite membrane, they achieved selectivities as high as 600 from a feed mixture that was only 0.5 mol % benzene. However, the fluxes in both cases were rather low. Wegner et al.34 reported that xylene isomers quickly fouled their MFI membrane during pervaporation and that, although the membrane showed initial (low) ideal selectivities, at long times (>60 h), the fluxes of the isomers appeared to approach the same value. No separation was obtained for any binary mixtures of xylenes. The fouling and lack of selectivity (ideal and separation) was attributed to the presence of nonzeolite pores. More recently, m- and p-xylene isomers were separated by pervaporation by a ferrierite membrane with selectivities greater than 16.35 Only p-xylene was detectable in the permeate, but the fluxes were low (∼10-9 mol m-2 s-1). Others have studied the vapor-phase permeation of aromatics through molecular sieve membranes. In this experimental setup, the isomers are vaporized into a carrier stream, typically helium, before being fed to the membrane. Baertsch et al.36 studied membranes containing a significant fraction of small nonzeolite pores and found that single-file diffusion dominated the transport of various aromatics through their silicalite-1 membrane, and hence, they could not separate mixtures even though p-xylene/o-xylene ideal selectivities were as high as 12. Keizer at al.7 successfully separated o-

and p-xylene with selectivities greater than 100 at 400 K using a silicalite-1 membrane composed mostly of zeolite pores and capable of separations by molecular sieving, but had difficulty quantifying their results because of the detection limits of the apparatus. Using a feed pressure of 0.3 kPa per isomer, they found that o-xylene did not block p-xylene from the membrane pores at low temperature and that the p-xylene flux exhibited a maximum with temperature because of the opposing effects of adsorption and diffusion. A more extensive study of the vapor permeation of the xylenes through an MFI membrane was recently performed by Xomeritakis et al.37,38 They studied the single-gas permeation of p- and o-xylene from 295 to 473 K and found that, although the p-xylene flux is a weak function of temperature, the flux of o-xylene exhibits a strong minimum at about 373 K. The presence of p-xylene in the feed greatly enhanced the flux of o-xylene over its single-gas flux. Also, as the partial pressure of p-xylene in the feed increased from 43 to 430 Pa, the selectivity dropped from 48 to 3. High selectivities were only observed at low partial pressures of the isomers. In addition, they reported long transients for the flux of the xylenes. p-Xylene required 24 h to reach a steady state at 373 K, and the introduction of o-xylene into the feed resulted in another 6 h of transient behavior. The current study characterized several molecular sieve and zeolite membranes for the separation of aromatic molecules, specifically xylene isomers. Novel membranes composed of SAPO-5, SAPO-11, and boronsubstituted ZSM-5 were studied, in addition to the mordenite, silicalite-1, and ZSM-5 structures. The SAPO structures were chosen because of their pore sizes. The larger pores (as compared to MFI) should allow larger fluxes while still exhibiting size selectivity. Because a mordenite membrane has successfully separated aromatics by pervaporation,31 one was tested for separation using vapor permeation. The MFI structures studied were synthesized by the recently reported alkali-free crystallization techniques of Tuan et al.9,39 These membranes exhibit separations by molecular sieving8,9,39 as opposed to the preferential adsorption separations reported by other researchers in this lab using different synthesis techniques.22,23,36,40,41 The permeation through the membranes was character-

568

Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001

ized as a function of both feed partial pressure and temperature. The different pathways for diffusion through the membranes were studied using transient experiments. The trends seen for the experiments were used to propose permeation mechanisms for the aromatic molecules. Experimental Methods Membrane Synthesis. Preparation. Because only a thin separating layer is required to form an effective membrane, the membranes are grown on mesoporous supports. Three support materials were used: R-alumina, γ-alumina, and stainless steel. The R- and γ-alumina tubes were obtained from U.S. Filter (0.68-cm i.d., 1-cm o.d., 4.7-cm length). The R-alumina tubes contained 200-nm pores, while the γ-alumina tubes had a 5-µm-thick layer, with 5-nm pores, on the inside edge of the support. About 1 cm on the end of each alumina support was glazed (IN 1001, Duncan) to prevent membrane bypass. The high temperatures required to set the glaze altered the 5-nm pores of the γ-alumina supports. The N2 flux (measured using a 138 kPa pressure drop) was 140 times larger through a support to which the glaze was applied after the γ-alumina layer (as done in this study) than a support to which the glaze was applied before the γ-alumina layer. The porous stainless steel supports were obtained from Mott Metallurgical Company (0.65-cm i.d., 0.95-cm o.d., 2.5-cm length) and consisted of 500-nm pores. Nonporous stainless steel tubes were welded onto each end of the supports to provide a sealing surface for the O-rings. The membranes were deposited on the supports through hydrothermal synthesis. One end of the support was sealed with Teflon tape and a Teflon endcap. The support was then filled with the synthesis gel in accordance with the procedure described below for each membrane type. The top end was taped and capped, and the support was sealed inside a Teflon-lined autoclave (Parr) and then placed in an oven. After the first layer was synthesized, the membrane was removed from the autoclave, cleaned, and dried, and the N2 permeance was tested using an imposed 138-kPa pressure drop. A continuous membrane should be impermeable to N2 prior to calcination. For permeable membranes, additional layers were synthesized until the membrane was impermeable. Once completed, the membranes were calcined in a temperature-controlled oven to remove the template molecules from the membrane pores. The oven was heated and cooled at 0.6 and 1.2 K/min, respectively, to minimize thermal stresses. Large Pore Membranes. SAPO-5. The SAPO-5 membrane, AFI1, was synthesized on an R-alumina support from a gel with a molar composition of 2.0 TPA:0.4 SiO2: 1.0 Al2O3:1.0 P2O5:50 H2O, where TPA is the tripropylamine template molecule and Ludox AS40 (40% SiO2, DuPont) was the silica source.42 Four synthesis layers were deposited at 460 K for 24 h each. The membrane was dried at 373 K between each layer, and calcined for 24 h at 870 K after the four layers were deposited. The high calcination temperature was required because, after calcination at 750 K, the normally white SAPO-5 crystals were brown, indicating the presence of carbonaceous material. SAPO-11. The SAPO-11 membrane, AEL1, was synthesized onto R-alumina supports using the procedure of Meriaudeau et al.,42 which is similar to that used for the SAPO-5 membrane. The synthesis gel had a molar

composition of 1.5 DPA:0.4 SiO2:1.0 Al2O3:1.0 P2O5:50 H2O, where DPA is the dipropylamine template molecule and Ludox AS40 was the silica source. Four synthesis layers were deposited at 460 K for 24 h each. The membrane was dried at 373 K between each layer and calcined for 24 h at 870 K once complete. Again, the high calcination temperature was used after initial attempts to calcine the membranes at lower temperatures failed to completely remove the organics, leaving the membrane crystals brown in color. Mordenite. Unlike the other membranes, the mordenite membrane, MOR1, was synthesized on a stainless steel support without a template molecule. The synthesis gel had a molar composition of 3.0 Na2O:1.0 Al2O3: 20 SiO2:200 H2O. A precrystallization layer was deposited at 450 K for 24 h prior to the membrane synthesis to reduce the pore size of the support and provide an anchor for the zeolite layer. Two membrane layers were deposited at 450 K for 48 h each to seal the membrane. The completed membrane was heated to 470 K for 16 h to remove any water from the membrane pores. Because no template molecule was used, calcination was not required. MFI Membranes. The silicalite-1 membrane, SIL1, was synthesized on a stainless steel support from an alkali-free gel with a molar composition of 19.46 SiO2: 438 H2O:1 TPAOH, where TPAOH is the tetrapropylammonium hydroxide template molecule (Aldrich). The ZSM-5 membrane, Z1, was synthesized on a stainless steel support from an alkali-free gel with an Si/Al ratio of 600. The molar composition of the synthesis gel was 438 H2O:19.5 SiO2:0.0162 Al2O3:1 TPAOH. The silicon source was Ludox AS40, and the aluminum source was aluminum isopropoxide (>98%, Aldrich). A detailed description of the synthesis procedures for both membranes is given by Tuan et al.39 Prior to synthesis, the support was boiled in water for 1 h and dried overnight to remove contaminants and loose particles. Once filled with the synthesis gel, the support was left overnight at room temperature to allow the gel to soak into the support. The tube was refilled, taped, capped, sealed in an autoclave, and placed in a 460 K oven for 22 h to crystallize the first layer. The first crystallization reduces the pore size of the support and provides seed crystals for the subsequent crystallization. A second layer was crystallized at 460 K for 48 h. The membranes were calcined for 8 h at 750 K. The B-ZSM-5 membranes were synthesized on stainless steel supports following the procedure of Tuan et al.9 The silica and boron sources were Ludox AS40 (DuPont) and boric acid (99.5%, Sigma), respectively. Membranes BZ1 and BZ2 were synthesized from a gel of molar composition 1.55 TPAOH:0.195 B(OH)3:19.46 SiO2:438 H2O. The synthesis procedure was similar to the alkali-free Al-ZSM-5 procedure.39 One end of the tube was taped and capped, and the gel was allowed to soak into the support overnight. The tube was refilled, plugged with a Teflon cap, and sealed in the autoclave. The precrystallization layer and subsequent membrane layers were synthesized at 460 K for 24 h. Both membranes required four synthesis layers. The membranes were calcined at 750 K for 8 h. Separation Apparatus. Aromatic mixtures were separated in a continuous-flow system that utilized a cross-flow permeation cell, described in detail elsewhere.40 The vapor feed flowed axially through the membrane tube, and the components permeated radi-

Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001 569

ally outward. Viton O-rings, resistant to aromatics, sealed the membrane into the module and prevented bypass over the sealed ends of the support. The aromatics were fed to the system as a mixture of liquids via a syringe pump. The feed liquid was vaporized into a preheated helium stream while passing through a heated zone maintained at a minimum temperature of 423 K. This combined vapor feed then flowed to the membrane module. On the permeate side of the module, a helium sweep gas continuously removed the permeation products from the external surface of the membrane. Helium flow rates for both the feed and the sweep were set at about 40 cm3/min at STP using mass flow controllers. A bypass line allowed the concentration of the aromatics in the feed to be measured. To prevent condensation of the organics, all system lines were wrapped with heating tape and insulated. The temperatures of the system lines were above 423 K. Two thermocouples were placed in the module feed entrance and permeate exit lines to monitor the stream temperatures. The permeate and retentate streams were analyzed with an HP 6890 gas chromatograph equipped with a flame-ionization detector. A 10-port pneumatic sampling valve was used to switch between the retentate and permeate streams. Initially, the chromatographic separation was accomplished using a 2-m Alltech AT-1200 packed column. For the higher-selectivity separations, a 15-m Alltech AT-1 megabore capillary column was required. Because of column limitations, m- and pxylene could not be separated. The volumetric flow rates of the retentate and permeate streams were measured at atmospheric temperature and pressure (typically 299 K and 84 kPa) using soap film flow meters. Procedure. The membranes were calcined for 4 h at 673 K to remove contaminants prior to each experimental run at a given liquid feed composition or temperature. After the membranes were sealed in the permeation module, they were purged with helium at 373 K before the flow of organics was started. For transient experiments, the permeation system was originally allowed to reach steady state in the module bypass configuration and was then switched to module flow to reduce the system time constant. Because of the analysis time involved, the resolution of those experiments was only 6 min, but because of the time scales involved with xylene permeation through the MFI membranes, this was often sufficient. Experiments at different temperatures were typically started in the middle of the explored range (∼430 K), increased to the maximum temperature, and then decreased to the lowest temperature because of the long transient responses seen for some membranes. Possible hysteresis effects were investigated by returning to the initial temperature but were not seen. For experiments at different partial pressures of xylenes, the syringe pump flow rate was started at the low value and incrementally increased over the experimental range. All reported selectivities were calculated as permselectivities, i.e., ratios of the permeances. The permeances were calculated as the fluxes divided by the partial pressure driving forces. Because the module has a cross-flow design, a logarithm of the mean pressure drop was used to calculate the driving force. Results Large Pore Membranes. SAPO-5. The steady-state fluxes of p- and o-xylene through the SAPO-5 mem-

Figure 2. Single-gas and mixture fluxes through membrane AFI1 at 373 K as a function of feed partial pressure

Figure 3. Single-gas and mixture fluxes of benzene, o-xylene, and p-xylene through membrane AEL1 as a function of temperature at a feed partial pressure of 8.4 kPa for each species.

brane, AFI1, are shown in Figure 2 as a function of the feed partial pressure at 373 K. All fluxes are linear functions of the feed pressure for the pressures used. p-Xylene permeates faster than o-xylene, with ideal selectivities between 1.7 and 2.0. In a 50/50 mixture, however, the fluxes of both isomers are the same and similar to, but slightly lower than, the o-xylene singlegas fluxes. SAPO-11. The single-gas fluxes of benzene, p-xylene, and o-xylene and the mixture fluxes of benzene and o-xylene are shown as a function of temperature in Figure 3 for the SAPO-11 membrane AEL1 at a feed pressure of 8.4 kPa for each isomer. All of the fluxes decrease with temperature. At 353 K, the single-gas fluxes for o-xylene and benzene are about equal, but as the temperature increases, the flux of o-xylene drops more rapidly than does that of benzene. The single-gas flux of p-xylene is lower at 353 K than those of the other aromatics, but as the temperature increases, it matches the flux of o-xylene. In the benzene/o-xylene mixture, the fluxes of both species are about the same. Mordenite. The fluxes of benzene, p-xylene, and m-xylene through mordenite membrane MOR1 were studied as a function of temperature for both single gases and mixtures at a feed pressure of 8.4 kPa per isomer (Figure 4). All of the fluxes, whether mixture or single gas, are almost independent of temperature over the range studied. Benzene permeates approximately 3 times faster than p-xylene as a single gas, but in a mixture, the benzene flux is lower and essentially the same as the p-xylene flux, which is the same as its single-gas flux. That is, p-xylene significantly inhibits

570

Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001

Figure 4. Single-gas and mixture fluxes of benzene, m-xylene, and p-xylene as a function of temperature for membrane MOR1 at a feed partial pressure of 8.4 kPa for each species. For mixture experiments, the second species is given in parentheses.

Figure 6. Mixture fluxes of o- and p-xylene through membrane Z1 as a function of temperature at a feed pressure of 2.1 kPa for each species.

Figure 5. Mixture fluxes of o- and p-xylene through membrane SIL1 as a function of temperature at a feed pressure of 2.1 kPa for each species.

Figure 7. Transient fluxes through membrane Z1 at 405 K at a feed pressure of 2.1 kPa for each species.

benzene permeation. In contrast, benzene permeates faster in a mixture with m-xylene than as a single gas, and the m-xylene flux in the mixture is larger than the single-gas flux of benzene. MFI Structures. Silicalite-1 and ZSM-5. Figure 5 shows the steady-state fluxes of p- and o-xylene through silicalite-1 membrane SIL1 for a mixture at a feed pressure of 2.1 kPa for each isomer. The membrane is selective for p-xylene over the entire temperature range, with a maximum selectivity of about 4.6 at 400 K. The permeance of p-xylene exhibits an inflection point near 415 K before increasing again at higher temperatures. The permeance of o-xylene increases slowly at low temperature but, beyond 450 K, exhibits a rapid increase. At the high temperatures, the rates of increase of the permeances for the two isomers are comparable. The mixture fluxes of p- and o-xylene through the ZSM-5 membrane Z1 at a feed pressure of 2.1 kPa for each isomer are shown in Figure 6. The permeance trends are similar to those for the silicalite-1 membrane. The maximum selectivity (5.5) and the inflection point for p-xylene permeance are at higher temperatures than the corresponding values for SIL1. The o-xylene permeance starts to increase rapidly at about the same temperature, 450 K. For both isomers, the permeances show a larger dependence on temperature for membrane Z1 than for SIL1. For the fluxes presented, the first samples were typically taken after 2 h to allow time to reach a steady state. The flux was considered to be at steady state when six consecutive analyses, over the course of 36

min, yielded similar results. However, for membrane Z1, the o-xylene flux was found to be at steady state even when measurements were taken before 2 h, whereas the p-xylene flux was still changing. Thus, in repeat experiments, the flux measurements were started when the membrane was first exposed to the feed. The transient mixture flux at 408 K through membrane Z1 is shown in Figure 7. The o-xylene flux quickly approaches a steady-state value, whereas the p-xylene flux remains low for the first hour. The p-xylene flux then increases dramatically after more than 1 h and reaches a steady-state value higher than that of the o-xylene. The sigmoidal shape of the transient p-xylene flux is similar to the breakthrough curves seen in packed-bed adsorbers.43 This long transient behavior differs from the results that other researchers report for lighter permeating species23,44 but agrees with the xylene vapor permeation results of Xomeritakis et al.38 and the xylene pervaporation results of Matsufuji.45 B-ZSM-5. The borosilicate ZSM-5 membranes had the highest selectivities for the xylene isomers. The single- and mixture-gas fluxes of the isomers through membrane BZ1 as a function of temperature are shown in Figure 8 at a feed pressure of 2.1 kPa per isomer. The temperature dependence is weaker than for the other MFI membranes, and except for the data point at 405 K, the flux of p-xylene in the mixture has the same general trend as previously seen and is the same order of magnitude. However, the o-xylene flux in the mixture for membrane BZ1 is an order of magnitude lower than the fluxes through the other MFI membranes. Also, the o-xylene flux exhibits a low-temperature minimum in

Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001 571

Figure 8. Single-gas and mixture fluxes of o- and p-xylene through membrane BZ1 as a function of temperature at a feed pressure of 2.1 kPa for each species.

Figure 10. Single-gas and mixture transient fluxes through membrane BZ1 at 405 K at a feed pressure of 2.1 kPa for each species.

Figure 9. Ideal and separation selectivities for membrane BZ1 as a function of temperature at a feed pressure of 2.1 kPa for each species

the mixture. At 405 K, the mixture flux of o-xylene is 2.5 times larger than the single-gas flux. Xomeritakis et al.38 reported a similar minimum for the single-gas flux of o-xylene through their MFI membrane. The single-gas fluxes for both isomers in Figure 8 exhibit the same general trends as for the corresponding xylene in the mixture but are higher. The ideal and separation selectivities for membrane BZ1 are shown in Figure 9 as a function of temperature. Although the ideal selectivities are twice as high as the mixture selectivities, both follow a similar trend. Both selectivities exhibit a maximum at around 430 K, with an ideal selectivity of over 130 and a separation selectivity of about 60. As the temperature increases, the selectivities decrease, but the membrane is selective for p-xylene at temperatures as high as 510 K, where the separation selectivity was 2.8. Figure 10 shows the single-gas and mixture transient fluxes of the xylene isomers through membrane BZ1 at 405 K and a feed partial pressure of 2.1 kPa for each isomer. For the single-gas experiments, p-xylene breaks through 1.5 h sooner than the o-xylene. In the mixture, both isomers appear earlier than they do as single gases and at the same time. This is in contrast to the behavior seen for membrane Z1 (Figure 7), in which o-xylene permeated the membrane initially and p-xylene permeated only at longer times. For membrane BZ1, the p-xylene flux approaches the same steady-state value in both single-gas and mixture experiments. However, the o-xylene flux is about a factor of 4 larger in the mixture than as a single gas.

Figure 11. (a) Transient fluxes of p-xylene in a mixture with o-xylene through membrane BZ2 at 373 and 405 K at a feed pressure of 0.9 kPa for each species. (b) Transient fluxes of o-xylene in a mixture with p-xylene through membrane BZ2 at 373 and 405 K at a feed pressure of 0.9 kPa for each species.

Figure 11a and b reports the transient fluxes for a mixture of p- and o-xylene at a feed pressure of 0.9 kPa per isomer at 373 and 405 K through membrane BZ2. The p-xylene flux (Figure 11a) displays the same trends as seen in membrane Z1 (Figure 7). The flux is small initially before breaking through and increasing to the steady-state value. The p-xylene breaks through in less time and reaches a higher steady-state value at the higher temperature. The o-xylene flux also matches the trends seen in membrane Z1. The flux at each temperature quickly increases to a maximum value, but only the flux at 373 K reaches a constant value. Although the maximum flux for o-xylene is higher at 405 K, it decreases, so that, after 11 h, the flux is lower at 405 K

572

Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001

Figure 13. Separation and ideal selectivities for p-xylene over o-xylene through membrane BZ2 as a function of temperature and feed partial pressure

Figure 12. (a) Single-gas and mixture fluxes for p-xylene through membrane BZ2 as a function of temperature and feed partial pressure. (b) Single-gas and mixture fluxes for o-xylene through membrane BZ2 as a function of temperature and feed partial pressure

than at 373 K. The time to the maximum o-xylene flux is much less dependent on temperature than it is for p-xylene. The steady-state mixture and single-gas fluxes for p-xylene through membrane BZ2 (Figure 12a) show a transition from one flux regime to another. At low pressures, the flux exhibits a maximum at about 405 K, followed by a minimum at about 465 K. At a feed pressure of 2.5 kPa, however, the flux of p-xylene increases linearly over the entire temperature range. The single-gas flux of p-xylene at 0.9 kPa follows the same behavior as the flux of p-xylene in the mixture with o-xylene at the same pressure. Figure 12b shows that the mixture and single-gas fluxes of o-xylene exhibit behavior different from that of the p-xylene fluxes. At higher pressures, the o-xylene flux increases over the entire temperature range, whereas at lower pressures, a slight minimum occurs at low temperature. The selectivities associated with Figure 12a and b are shown in Figure 13. The highest selectivities are obtained at the lowest xylene feed pressures. The general trends are the same as those in Figure 9 for membrane BZ1, but the selectivities are lower. The ideal selectivity measured at a pressure of 0.9 kPa is larger than the corresponding mixture selectivity but is similar to the separation selectivity at a pressure of 0.4 kPa. Discussion Large Pore Membranes. SAPO-5. The behavior exhibited in Figure 2 for membrane AFI1 indicates single-file diffusion.36,46 The molecules are unable to pass one another in the mixture because of the confines

of the pores, and the more rapidly diffusing species is inhibited by the slower species, so that this membrane does not separate the xylene isomers. Although SAPO-5 catalyzes aromatic reactions, most notably isomerization,47 adsorption data for aromatic hydrocarbons on SAPO-5 are not readily available. A linear adsorption isotherm (Henry’s law region) for both xylene isomers on SAPO-5 at 373 K and the 2-12 kPa range would explain the linear dependence of flux on feed pressure. Also, because the AFI framework consists of a straight, one-dimensional pore structure, singlefile diffusion would be the expected transport mechanism because of the restricted pore size (no intersections where the molecules could pass each other). If the adsorption isotherms for both isomers were similar, then single-file diffusion would result in the same flux for both isomers in the mixture, as each isomer would be equally likely to enter the pore. However, Gump et al.41 found a linear dependence on feed pressure for n-hexane flux through nonzeolite pores (intercrystalline regions and grain boundaries) in ZSM-5 membranes. Similar pores might cause the linear dependence of flux on pressure in membrane AFI1. Although pores of this type in MFI membranes separate alkane isomers by preferential adsorption and pore packing by the linear isomer,41,48 they are unable to separate aromatic mixtures.36 The difference in molecular size and adsorption properties for xylene isomers is too small for such a mechanism to be selective. The same could be true in non-SAPO pores in the SAPO-5 membrane, and because the molecules are unable to pass one another in the nonSAPO pores, single-file diffusion dominates the transport. SAPO-11. Membrane AEL1 also exhibits behavior that would be expected for single-file diffusion; benzene in the mixture is slowed by o-xylene so that the two molecules have almost the same flux. Interestingly, p-xylene does not permeate the membrane faster than o-xylene, even though the oval pores of the structure should discriminate between the isomers on the basis of size. The membrane might therefore consist mostly of nonzeolite pores, similarly to membrane AFI1. However, the decrease in flux with increasing temperature indicates that adsorption plays an important role in transport. As the temperature increases, the surface coverage decreases. The decrease in adsorbed concentration is not compensated for by an increase in the diffusion rate, and the flux decreases. Some models of permeation through molecular sieve membranes have included both Maxwell-Stefan surface

Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001 573

diffusion and activated gaseous transport.49,50 In surface diffusion, molecules permeate the membrane as an adsorbed phase on the pore walls. Therefore, increasing the partial pressure of the permeating species in the feed has diminishing returns: once the surface is saturated, increasing the partial pressure does not increase the flux. For activated gaseous diffusion, permeating molecules retain a gaseous character in the pores of the membrane.51 Increasing the temperature increases the diffusion rate. In the range of temperatures studied, the flux decreases with increasing temperature for membrane AEL1. Therefore, activated gaseous transport is unlikely to contribute significantly to the overall transport in this membrane. Because of the lack of adsorption data, the flux behavior cannot be predicted using the Maxwell-Stefan equations. However, they qualitatively predict that an increase in temperature can decrease the flux.49,50 This occurs when the decrease in coverage dominates the increase in diffusion at higher temperatures. The permeation of pand o-xylene through this membrane appears to be governed by surface diffusion, but the transport could be through either SAPO or non-SAPO pores. Because the flux of aromatics through nonzeolite pores in MFI membranes increased with temperature,36,46 similar behavior might be expected for non-SAPO pores. Therefore, the decreasing flux of aromatics through AEL1 might indicate that transport is through SAPO pores. Mordenite. Transport across membrane MOR1 is independent of temperature in the range studied and exhibits some small ideal selectivity (∼1.8) for the benzene/p-xylene system. The membrane also separates mixtures of benzene and m-xylene with low selectivities (